The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the generation of magnetic field gradients for use in fast pulse sequences.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins, and after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Most NMR scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts. There is a class of pulse sequences which have a very short repetition time (TR) and result in complete scans which can be conducted in seconds rather than minutes.
For example, the concept of acquiring NMR image data in a short time period has been known since 1977 when the echo-planar pulse sequence was proposed by Peter Mansfield (J. Phys. C.10: L55-L58, 1977). In contrast to standard pulse sequences, the echo-planar pulse sequence produces a set of NMR signals for each RF excitation pulse. These NMR signals can be separately phase encoded so that an entire scan of 64 views can be acquired in a single pulse sequence of 20 to 100 milliseconds in duration. The advantages of echo-planar imaging ("EPI") are well-known, and there has been a long felt need for apparatus and methods which will enable EPI to be practiced in a clinical setting.
One of the main limitations in applying the EPI pulse sequence and other fast pulse sequences in a clinical setting is the inability of commercially available MRI systems to produce the required magnetic field gradient pulses. Fast pulse sequences require very short duration magnetic field gradient pulses which in turn require a very high risetime in the gradient coil current. Methods used to achieve higher current risetime include reducing gradient coil inductance by employing small local coils, and increasing gradient amplifier voltage and power. The latter solution requires an increase in voltage of ten times and results in a proportional increase in gradient amplifier cost.
A number of methods have been used to increase gradient amplifier voltage without proportionally driving up its cost. In one scheme this has been accomplished with the addition of charged capacitors and switching networks to the existing amplifiers that resonate the coil inductance and rapidly move the coil current to the desired level. Sinusoidal pulses or "trapezoids" with sinusoidal transitions may be generated with this technique. Another technique uses a much larger charged capacitor inside of a full bridge switching network to apply an almost constant voltage to the coil to generate fast ramps and freewheel the current during the flat portion of the trapezoid. In both of these methods the circuitry is partitioned such that the existing gradient amplifiers supply the electrical losses of the system while the added high voltage circuitry supplies the reactive power to the coil inductance which is recovered back into the capacitor at the end of each current pulse. Each technique requires some method of managing the energy flow so the capacitor voltage starts each pulse at a controlled level.
Both of the above techniques impose undesirable waveform restrictions on the gradient pulse shape because the voltage on the coil is determined by the capacitor. It is the object of this invention to remove these restrictions while retaining the benefits of dual power amplifiers, one optimized for supplying low voltage losses and the other optimized for supplying the high voltage reactive power.